Plants host distinct microbial communities on and inside their tissues known as the plant microbiota. The interplay between members of the plant microbiota and host plants ranges from mutualistic to commensalistic and pathogenic relationships.
The endophytes are the symbionts that reside in the plant tissues for at least the part of their life cycle. These symbionts play a pivotal role in the ecological and evolutionary success of the plant.
The celebrated rhizobium, a soil bacterium makes sure that dinitrogen from the atmosphere is being efficiently converted to ammonia and is beneficial for the leguminous plants such as peas, beans, alfalfa, etc. It also produces the nitrogen-containing alkaloids which keep herbivores at bay.
Another example of Codonopsis pilosula, a medicinal herb, aka poor man’s ginseng, has some strains of Bacillus subtilis which aid in the improvement of plant size, chlorophyll content, and photosynthesis. Populus trichocarpa hosts the Stachybotrys which reduces the severity of leaf rust caused by the fungus Melampsora.
In Alfalfa (Medicago sativa) anchors the gram-negative bacteria, Sinorhizobium meliloti which not only fixes the nitrogen but also helps the plant to withstand freezing conditions.
Apparently, in this little-known world of the plant microbiome, plenty of gems are undiscovered. Plants acquire many of these microbes from soil and air, while some are passed vertically from generation to generation by seeds. Plant microbes can be beneficial, destructive, or a bit of both.
The friendly ones assist with survival in the time of adversity, enhance the photosynthesis, helps in achieving better growth and yield. All this opens the gates for the possibility of having a second plant industry boosting wave: plant partner revolution, wherein the possibilities using these potential microbes for the crop improvement can be explored and outline the modus operandi.
“We’re going to need to increase yield,” says Posy Busby, an ecologist at Oregon State University in Corvallis.
“If we can manage and manipulate microbiomes … this could potentially represent an untapped area for increasing plant yield in agricultural settings.” This gave birth to the new start-ups and where such research is going on and novel microbial treatments are being devised.
“The last five years have seen an explosion in this,” says Dangl, who cofounded AgBiome, which soon plans to market a bacterial treatment that combats fungal diseases. Already well-settled agricultural companies like Bayer AG are investing millions of dollars in potential microbial treatments.
Propitiously, one such analysis has been done on Cacao plants (Theobroma cacao) by Natalie Christian, a graduate student at Indiana University Bloomington. The plant is being constantly challenged by the fungi namely, Phytophthora palmivora which causes black rot on the cacao pods. But the researcher found out that the savior fungus: Colletotrichum tropicale comes to the rescue of the plant.
During the observation period, the sixth sense of Natalie kicked in and she had the hunch that the prime source of young cacao tree’s microbiome might be the dead and decaying leaves on the rainforest or orchard floor. Soon she started with her experiment to know what kind of benefits the leaf litter offers.
She raised fungus-free cacao seedlings in the laboratory incubator till they grew half a meter tall, and then placed them in pots outside, and surrounded some with leaf litter from the healthy cacao tree, some with leaf litter from other trees and some with no litter at all. After two weeks, she brought them back to the lab and checked for the microbes and corroborated that the microbial population differed between the litter treatments and stated that the plants with litter had less diversity than that of without any litter.
In another experiment which she did to see which set of the plants had maximum immunity to the black pod rust. She rubbed a bit of rot on the leaves of the plants in each group. Three weeks later, the size of lesion on the plants surrounded by the cacao litter was smallest, those with litter from other trees had slightly bigger lesions, and the ones with no litter had lesions double the size of mixed litter plants.
“Getting exposed to the litter of their mother or their own kind had a very strong beneficial effect on the resistance of these young plants,” says plant biologist Keith Clay of Tulane University in New Orleans, a co-author of the study. Mejía reported in 2014 in Frontiers in Microbiology that C. tropicale causes cacao to turn on defensive genes and fight off the rot. The Cacao farmers tend to clear leaf litter out of the orchards whereas this study suggests otherwise.
On account of another expedition of Geoff Zahn and Anthony Amend narrates the plight of Phyllostegia kaalaensis, members of the mint family which is near to extinction. The reason for it being critically endangered was both habitat loss and powdery mildew. These plants native to Oahu, Hawaii survive only in two government-managed greenhouses.
Zahn desperately wanted to save the P.kaalaensis and started to look out for the options, first, he thought of antibiotics, but then over-ruled it due to probable damage to gut flora, rendering a person vulnerable to harmful microbes. To devise the beneficiary approach, he pondered and came to the conclusion that the fungicide sprayed on the saplings may make them devoid of the microbe required for the protection. Therefore, he came with the idea of Probiotics to regain the plant’s immunity which could help plants to thrive in a wild environment.
For this, the keen researcher took a P. kaalaensis cousin, Phyllostegia hirsuta, which had the ability to survive in wild. Zahn made a slurry of the P.hirsuta leaves and sprayed on the P.kaalaensis saplings and fortuitously these saplings experienced mild infection compared to untreated plants. The probiotic had worked.
Later, Zahn did DNA sequencing and identified a yeast called Psuedozyma aphidis, which lives on the leaves and passively absorbs the nutrients from the environment, but as soon as mildew spores land nearby, the yeast grows tentacle-like filaments which girdle and feed on mildew. Emboldened by the results Zahn planted six slurries sprayed saplings on the Oahu mountain, which lived for two years.
Compendium for all this plant microbiota can be used for various welfares purely depending upon the need of the time. These aforementioned findings are prospective herald for the next big things for plant microbiomes in agriculture as well as conservation. The need of the hour is to tend to microscopic helpers rather than manipulating the genome of the plants.
